British Patent Specification No. GB-A-1494706 describes an alignment arrangement between a tube socket and an artificial limb element that comprised two oppositely facing concentric part-spherical convex surfaces provided on the end of the socket. The surfaces were engaged by complementary concave surfaces provided respectively within the limb element and within a clamping ring. Clamping screws passed through plain holes in the ring into threaded holes in the limb element, tightening thereof inducing a controlled degree of frictional resistance between the part-spherical surfaces. At least one pair of jacking screws passed through threaded holes in the ring on at least one diameter thereof to engage abutments on the socket. The arrangement enabled an angular alignment between the socket and the limb element set by operation of the jacking screws to be maintained by pure friction between the part-spherical surfaces. The above device has proven suitable for manufacture in light alloy and has also proven a success in service, but it is complex and it suffers from the problem that all the load passes through the clamping ring, which is relatively high stressed.
A further kind of alignment device for an artificial leg comprises a generally circular male member having a head formed with a multiplicity of recessed surfaces disposed at equi-angular intervals about the axis of the head and a body of larger diameter than the head and having a smoothly curved outer surface. A female member has a central bore into which the head of the male member fits and a multiplicity of screws in threaded bores in the female member disposed at equi-angular intervals and directed away from the entrance of the central bore. Accordingly, as the screws are tightened against the recessed surfaces the outer surface of the body is drawn into engagement with a complementary seat at the mouth of the bore. An arrangement of the above kind having four clamping screws is manufactured by Otto Bock in British Patent Specification No. 1307919 and one having three clamping screws is described in Patent Specification No. EP-A-0054391 (Robert Kellie & Son). Such an alignment device is of adequate strength when made in steel, but we have found that devices of this kind do not give adequate strength when made in light alloy, as is increasingly the practice in the prosthetic limb industry.